An analog switch is used to alternately pass or block a voltage signal which is allowed to vary with time. The switching element can be a transistor, e.g., a field effect transistor (FET) such as a metal-oxide-semiconductor field-effect transistor (MOSFET). The control input to the switch is typically a standard complementary metal-oxide-semiconductor (CMOS) or transistor-transistor logic (TTL) input, which is shifted by internal circuitry to a suitable voltage for changing the state of the switch from a non-conducting state to a conducting state or vice versa. For a typical switch, a logical voltage of 0 on the control input causes the switch to have a high resistance, so that the switch is OFF (non-conducting), and a logic 1 on the input causes the switch to have a low resistance, so that the switch is ON (conducting).
One type of bidirectional analog switch requires two discrete power switches (e.g., MOSFETs) to be placed back-to-back in series, with either a common source or a common drain. The total on-resistance of the switch is then twice that of an individual power MOSFET. An example of such an arrangement is shown in FIG. 1, wherein MOSFETs 10 and 11 are connected in a common source configuration. The conventional bidirectional analog switch has high drain to source resistor (Rds) and high capacitance in the on state (Con). As seen in FIG. 1, the signal can come from either side. When there is no signal the current can be blocked with back to back body diodes with the switches off. However, this results in high resistance because two switches are in series and have high capacitance.
Typically, a CMOS switch uses a PMOS (p-channel MOS) and an NMOS (n-channel MOS) in parallel across a Switch input (SWIN) and Switch output (SWOUT). In typical MOSFETs, the body is shorted to the source. Since the switch is bi-directional, the polarity of the input and output can flip, and thus the source and the drain of each MOSFET can likewise flip, or have reversed voltages (the use of PMOS and NMOS in parallel ensures that at least one of the MOSFETs will be on since the effective gate voltage is taken with respect to the source). A typical analog switch has CMOS body diodes connected to their sources when the switch is on.
In some prior art bidirectional switches, a body snatching scheme is employed in which the body region of a MOSFET is connected (snatched) to whichever side is currently the source, either SWIN or SWOUT.
U.S. Pat. No. 6,590,440 discloses a bidirectional battery disconnect switch including a four-terminal n-channel MOSFET having no source/body short and circuitry for assuring that the body is shorted to whichever of the source/drain terminals (T3 or T4) of the MOSFET is biased at a lower voltage. As shown in FIG. 2A, a battery disconnect switch S6 includes switch n-channel MOSFET M1 and body bias generator 50. A terminal T3 is connected to the negative terminal of a battery and a terminal T4 is connected to a load or battery charger, as shown in FIG. 2B. Body bias generator 50 includes a first pair of MOSFETs M2 and M3 and a second pair of MOSFETs M4 and M5. MOSFET M2 is connected between the drain and body of MOSFET M1, and MOSFET M3 is connected between the source and body of MOSFET M1, with the source terminals of MOSFETs M2 and M3 being connected to the body of MOSFET M1. MOSFETs M2 and M3 contain a conventional source-body short. The gate of MOSFET M2 is connected to the source of MOSFET M1, and the gate of MOSFET M3 is connected to the drain of MOSFET M1. MOSFETs M4 and M5 are connected in parallel with MOSFETs M2 and M3, respectively. The gate terminals of MOSFETs M4 and M5, however, are connected in common to the body of MOSFET M1. The source and body terminals of MOSFETs M4 and M5 are shorted in the conventional manner, and shorted to the body of MOSFET M1. MOSFETs M2 and M3 function to short the body of MOSFET M1 to whichever of the source and drain terminals of MOSFET M1 is at a lower voltage. MOSFETs M4 and M5 function to prevent the body of MOSFET M1 from “floating” upward to an excessive degree when MOSFETs M2 and M3 are both turned off MOSFET M2 functions to short the drain and body of MOSFET M1 when the voltage at the drain is lower than the voltage at the source of MOSFET M1, and MOSFET M3 functions to short the body and source of MOSFET M1 when the voltage at the source is lower than the voltage at the drain of MOSFET M1. Thus, the body of MOSFET M1 is clamped to whichever of the drain and source terminals of MOSFET M1 is biased most negatively such that the source and the drain of the MOSFET M1 flip accordingly. In this case, the body snatches to NMOS source dynamically.
In FIG. 2B, a cascode N-channel MOSFET M6 is connected into the circuit. MOSFET M6 is a four-terminal device with no source/body short. The source/drain terminals of MOSFET M6 are connected to the source of MOSFET M1 and gate of MOSFET M2, respectively; the body of MOSFET M6 is connected to the body of MOSFET M1; and the gate of MOSFET M6 is connected to the positive terminal of battery B, a fact which makes it advantageous to implement body bias generator 50 inside the control IC. In this case, the body voltage is clamped below the voltage drop of one diode.
However, the applications for these switches often require fast signal frequency, which means that the capacitance should be low. To do this, the switching devices need to be made small. However, smaller devices tend to have higher drain to source resistance in the ON state Rdson. The high Rdson means that a relatively high drain to source voltage results across the MOSFET when the MOSFET is ON. The relatively high voltage across the MOSFET can turn on an internal body diode (if it goes over typical diode forward voltage drop ˜0.7 V). This is highly undesired because it results in loss of control of the MOSFET, and can cause latch up. In addition, the switch shown in FIGS. 2A-2B has slower switching speeds because of the capacitance inherent to body snatching MOSFETs. Furthermore, if the MOSFET M1 on-resistance Rdson is too low, then there is insufficient gate bias voltage to snatch the body. It is within this context that embodiments of the present invention arise.